364
10 Proteins of Muscle and the Cytoskeleton
and aAI and aB 1 have been phosphorylated
post-translationally. In theory, more than 100 different a crystallins could arise in one individual
by variations in the molecular size and chain composition. The ~ crystallins of the rat occur as at
least ten chain variants, of which four are primary
gene products; seven variants of y crystallins are
known, of which only one arises by posttranslational modification [239]. To the speciesspecific and individual differences one may also
add ontogenetic alterations. In the rat, all the
crystallin genes are active during early embryogenesis and they are then switched off at various
moments during further development. Only two
genes are later reactivated: the aB gene on day 18
and the ~s(Ys) gene post-natally [1]. In mammals,
y crystallin is the first to be produced in embryo
development, whereas in birds it is 0 crystallin; in
both cases, a and ~ crystallins later predominate.
This leads to differences in the crystallin spectrum
between different layers of the lens.
The four main classes of crystallins belong to
three protein super-families (a, Wyand 0). The
homology of ~ and y crystallins clearly ensues
from the similarity of their amino acid sequences
and the corresponding subdivision of their polypeptide chains into two domains, each with two
structural motifs. The sequence similarities of
37 % for the ~ and y chains of the rat, and 29 %
for the bovine ~Bp and yll chains are highly significant [61]. The amino acid sequences of the a
and 0 crystallins show no similarity to each other
or to the ~ and y crystallins, although the hydropathy profiles of the different chains have a similar pattern. The exonlintron structures of members of the three super-families are also quite different. Comparisons of orthologous sequences
from different species suggest that the rate of
evolution of all crystallins is very low (see Table 4.12, p. 161). Other lens proteins are also quite
conservative, e.g. the lens fibre proteins, which
probably arise from the gap junctions, have very
similar peptide cleavage patterns from the
amphibians to the mammals [280]. The maintenance of maximum transparency through closely
packed protein molecules probably requires specific spatial relationships between the side-chains
of the polypeptide chains, and this presumably
restricts the possibilities for amino acid substitution.
As crystallins are found only in the vertebrates
it may be assumed that they originated from nonlens proteins. In fact, a partial fragment of 76
amino acids from the aA crystallin of various
mammals shows significant sequence agreement
with the small heat-shock protein (hsp26) of Drosophila, Caenorhabditis and soybean. There is
evidence of a relationship between the Wy crystallins and the Ca 2 + -binding protein S of the bacterium Myoxococcus xanthus [110, 134]. The
taxon-specific crystallins are all related to, or are
even identical to, enzymes of cell metabolism: 0
with arginine succinate lyase, f with lactate
dehydrogenase B4, ~ with alcohol dehydrogenase,
' Y]
with aldehyde dehydrogenase, A with
hydroxyacyl-CoA-dehydrogenase, Q with aldose
reductase and aldehyde reductase, and 1: with aenolase. The crystallins 0, f and 1: are even catalytically active [120, 312].
The characteristically high molecular weight a
crystallins (Table 10.3) consist of two chain types,
aA and aB, which differ in about 40 % of their
170-175 amino acids. The relative proportion of
the two types varies from 9: 1 in the kangaroo to
4: 1 in the shark. The amino acid sequences of the
aA chains are now known for more than 40 mammals, 21 birds, 2 reptiles, 2 amphibians, and the
shark Squalus acanthias (Fig. 10.12); the aB
chains are known for only a few mammals.
Because there is apparently only one aA gene,
the aA sequences are orthologous and therefore
suitable for the study of taxonomic relationships;
genealogical trees of the aA sequences have, to
date, always been constructed on the maximum
parsimony principle and are thus subject to the
usual criticism of this method. However, the data
suggest an average rate of evolution of the aA
chains of about only 3 % per 100 million years.
The human and bovine aA chains differ at only
ten positions and the aB chains at only three;
thus, the rate of evolution of the aB chains is
probably even lower than that of aA. As a result
of the slow evolution of the a crystallins, even
distantly related species have identical aA
sequences, e.g. dog/cat, rat/golden hamster/the
gerbil Meriones ungulatus, giraffeihippopotamus,
and guinea-pig/springhaas Pedetes caffer [119].
The conservative nature of the lens proteins is
also apparent from immunological data. Of the
antigen determinants of undissociated bovine a
crystallin, 42 % are present in the agnathans, a
further 11 % in the cartilaginous fish, and a further 11 % in the bony fish, 21 % in the amphibians and reptiles, and only 15 % are restricted to
the mammals.
Three different a-crystallin chains are produced in the mouse (aA, aA ins and aB), although
there are only two genes each with three exons.
The 1376-bp first intron of the A gene, between
codons 63 and 64, contains 288 bp downstream
10 Proteins of Muscle and the Cytoskeleton
and aAI and aB 1 have been phosphorylated
post-translationally. In theory, more than 100 different a crystallins could arise in one individual
by variations in the molecular size and chain composition. The ~ crystallins of the rat occur as at
least ten chain variants, of which four are primary
gene products; seven variants of y crystallins are
known, of which only one arises by posttranslational modification [239]. To the speciesspecific and individual differences one may also
add ontogenetic alterations. In the rat, all the
crystallin genes are active during early embryogenesis and they are then switched off at various
moments during further development. Only two
genes are later reactivated: the aB gene on day 18
and the ~s(Ys) gene post-natally [1]. In mammals,
y crystallin is the first to be produced in embryo
development, whereas in birds it is 0 crystallin; in
both cases, a and ~ crystallins later predominate.
This leads to differences in the crystallin spectrum
between different layers of the lens.
The four main classes of crystallins belong to
three protein super-families (a, Wyand 0). The
homology of ~ and y crystallins clearly ensues
from the similarity of their amino acid sequences
and the corresponding subdivision of their polypeptide chains into two domains, each with two
structural motifs. The sequence similarities of
37 % for the ~ and y chains of the rat, and 29 %
for the bovine ~Bp and yll chains are highly significant [61]. The amino acid sequences of the a
and 0 crystallins show no similarity to each other
or to the ~ and y crystallins, although the hydropathy profiles of the different chains have a similar pattern. The exonlintron structures of members of the three super-families are also quite different. Comparisons of orthologous sequences
from different species suggest that the rate of
evolution of all crystallins is very low (see Table 4.12, p. 161). Other lens proteins are also quite
conservative, e.g. the lens fibre proteins, which
probably arise from the gap junctions, have very
similar peptide cleavage patterns from the
amphibians to the mammals [280]. The maintenance of maximum transparency through closely
packed protein molecules probably requires specific spatial relationships between the side-chains
of the polypeptide chains, and this presumably
restricts the possibilities for amino acid substitution.
As crystallins are found only in the vertebrates
it may be assumed that they originated from nonlens proteins. In fact, a partial fragment of 76
amino acids from the aA crystallin of various
mammals shows significant sequence agreement
with the small heat-shock protein (hsp26) of Drosophila, Caenorhabditis and soybean. There is
evidence of a relationship between the Wy crystallins and the Ca 2 + -binding protein S of the bacterium Myoxococcus xanthus [110, 134]. The
taxon-specific crystallins are all related to, or are
even identical to, enzymes of cell metabolism: 0
with arginine succinate lyase, f with lactate
dehydrogenase B4, ~ with alcohol dehydrogenase,
' Y]
with aldehyde dehydrogenase, A with
hydroxyacyl-CoA-dehydrogenase, Q with aldose
reductase and aldehyde reductase, and 1: with aenolase. The crystallins 0, f and 1: are even catalytically active [120, 312].
The characteristically high molecular weight a
crystallins (Table 10.3) consist of two chain types,
aA and aB, which differ in about 40 % of their
170-175 amino acids. The relative proportion of
the two types varies from 9: 1 in the kangaroo to
4: 1 in the shark. The amino acid sequences of the
aA chains are now known for more than 40 mammals, 21 birds, 2 reptiles, 2 amphibians, and the
shark Squalus acanthias (Fig. 10.12); the aB
chains are known for only a few mammals.
Because there is apparently only one aA gene,
the aA sequences are orthologous and therefore
suitable for the study of taxonomic relationships;
genealogical trees of the aA sequences have, to
date, always been constructed on the maximum
parsimony principle and are thus subject to the
usual criticism of this method. However, the data
suggest an average rate of evolution of the aA
chains of about only 3 % per 100 million years.
The human and bovine aA chains differ at only
ten positions and the aB chains at only three;
thus, the rate of evolution of the aB chains is
probably even lower than that of aA. As a result
of the slow evolution of the a crystallins, even
distantly related species have identical aA
sequences, e.g. dog/cat, rat/golden hamster/the
gerbil Meriones ungulatus, giraffeihippopotamus,
and guinea-pig/springhaas Pedetes caffer [119].
The conservative nature of the lens proteins is
also apparent from immunological data. Of the
antigen determinants of undissociated bovine a
crystallin, 42 % are present in the agnathans, a
further 11 % in the cartilaginous fish, and a further 11 % in the bony fish, 21 % in the amphibians and reptiles, and only 15 % are restricted to
the mammals.
Three different a-crystallin chains are produced in the mouse (aA, aA ins and aB), although
there are only two genes each with three exons.
The 1376-bp first intron of the A gene, between
codons 63 and 64, contains 288 bp downstream